1. Field of the Invention
This invention relates to systems and methods for delivering energy to tissue, and more particularly to systems for hyperthermic treatment or ablation of targeted tissues, such as tumors and the like. The system of the invention maintains a selected energy delivery profile in a targeted tissue volume to effectively localize thermal effects for a selected time interval.
2. Description of the Related Art
In recent years, a number of instruments have been disclosed for localized thermally-mediated treatments or ablations of tumors or other targeted tissues in an interior of a patient's body. Any such percutaneous or minimally invasive treatment offers the advantage of causing less damage to healthy tissue when compared to an open surgical procedure, for example an excision of a tumor. Further, a localized thermal treatment of a tumor can prevent seeding of the tumor which is believed to be a risk factor in an open surgery.
Several terms have been used to describe such thermally-mediated treatments, generally depending on the temperature range of the therapy, including terms such as hyperthermia, thermotherapy and ablation. Hyperthermia often is used to describe therapies that cause tissue temperatures in the range of 37° C. to about 45° C. or higher that do not cause immediate cell disruption and death. The term ablation typically describes temperature ranges that denature proteins, such as in coagulation, for example in the 50–100° C. range and higher. This disclosure relates to the controlled application of energy to tissue in any thermotherapy, and will typically use the terms thermally-mediated therapy or ablation to describe the methods of the invention that cover temperature ranges from about 37° C. to 200° C.
An exemplary thermally-mediated therapy of the invention is the ablation of tumors, whether benign or malignant, for example tumors of the liver. In a prior art therapy, heat has been applied to a tumor by means of direct contact of the targeted tissue with an exposed radio-frequency (Rf) electrode carried at the distal end of a insulated needle-type probe as depicted in FIG. 1A (see, e.g., U.S. Pat. No. 5,507,743). The principal problem related to the use of Rf electrode needles is that the tissue volume elevated in temperature is not adequately controlled and localized. For example, it may be desirable to maintain a targeted tissue region between 65° C. and 70° C. for 300 seconds. FIG. 1A illustrates the active heating of tissue around the needle electrode at time T1 which comprises a time interval just after the initiation of mono-polar Rf flow through the tissue (ground pad not shown). The arrows in FIG. 1A depict the application of Rf energy fairly deep into the tissue volume. Next, FIG. 1B illustrates that the active heating of tissue at time T2 around the electrode, which is limited in depth as indicated by the arrows. In a typical treatment with a fine needle, the initial active Rf energy will dehydrate or even desiccate tissue around the needle, and probably coagulate microvasculature. The result can be an elevation of the tissue's impedance (due to lack of fluid in the tissue) that is not altered by migration of body fluids to the site. Thus, even if Rf power delivery to the tissue is modulated by a feedback mechanism, such as impedance monitoring, the lack of the fluid content in the tissue may never allow substantial deep active Rf energy in the tissue volume around the electrode.
What is needed is a system and method for delivery of Rf energy to targeted tissue volumes in a precisely controlled manner for localization of thermal effects. It would desirable to provide an Rf system that can maintain a selected tissue temperature, and Rf density in tissue, independent of changes in voltage or current and without the need for feedback mechanisms.